The present invention relates to clutches employed with vehicle transmissions, and more particularly to preventing overheating of such clutches.
Dry clutches connected between the engine output and the transmission input are conventionally employed with vehicles having manual transmissions. The engagement of these clutches are controlled by a vehicle operator pressing on or releasing a clutch pedal. While these conventional manual transmissions have some drawbacks relative to conventional automatic transmissions—the need for the vehicle operator to actuate the clutch pedal and manually shift gears—they are still employed due to some inherent advantages. Namely, a conventional clutch and manual transmission is typically less expensive than a conventional automatic transmission and torque converter, and the conventional manual transmission arrangement does not have the energy losses associated with the torque converter.
Consequently, attempts have been made to develop a one or two clutch and manual type of transmission arrangement that will operate like an automatic transmission—an automated manual transmission or a powershift transmission. That is, employ a vehicle controlled clutch and gear shifting system, but without a torque converter or the more complex planetary gear sets and shift mechanisms of a conventional automatic transmission. One significant concern with these new systems, however, is the potential for heat build-up in the clutch while the vehicle is in a hill hold mode (i.e., the transmission is in a forward gear and the vehicle operator uses the accelerator pedal to maintain the vehicle at a stand still on a hill). In order for the engine to be running while the vehicle is in gear and not moving, there must be slippage somewhere along the drive line. In the conventional automatic transmission arrangement, the torque converter allows for this slippage by shearing the fluid therein, with the torque converter readily having the thermal capacity to absorb and dissipate the excess heat generated. But for an automated type transmission with a clutch instead of a torque converter, the clutch will create the slippage, which creates heat build-up in the clutch. This heat build-up can occur relatively quickly, and can approach temperatures that may cause significantly increased wear and possibly damage the clutch.
To overcome the heat build-up concern, some have devised audible or visual warning systems to alert the vehicle operator if the clutch temperature is too high. But the vehicle operator may ignore the warnings. Others have suggested the addition of one-way clutches or other transmission hardware to accomplish a hill hold function, but these types of solutions add to the cost and complexity of the transmission. Another suggested method for overcoming this concern is to rapidly and automatically engage and disengage the clutch to cause the drivetrain to pulsate in an attempt to encourage the vehicle operator to release the accelerator pedal and actuate the brake pedal. However, such pulsation being felt throughout the vehicle may cause concern for both the vehicle operator and other occupants. Yet another suggested method that attempts to overcome this concern is one where, above a certain temperature, the clutch will fully engage, thus causing the vehicle to move forward and the vehicle operator to engage the brake pedal to stop the forward movement. But this method may also cause a concern for the vehicle operator or vehicle occupants.
Thus, it is desirable to have an automated manual transmission with a hill hold function that prevents overheating of a clutch while avoiding many of the drawbacks of the prior art.